Worm gear burnishing machine and method



.1. M. CHRISTMAN 2,062,700

WORM GEAR BURNISHING MACHINE AND METHOD Filed Nov. 6, 1935 3 Sheets-Sheet l 1936. m. CHRIS'TMAN 2 062,700

WORM GEAR BURNISHING MACHINE AND METHOD Filed NOV. 6 1935 3 Sheets-Sheet I x-llll/m/ Dec. 1, 1936. J. M. CHRISTMAN I WORM GEAR BURNISHING MACHINE AND METHOD Filed Nov. 6, 1935 3 Sheets-Sheet s .IIIIIIIIl/l/l/ -IIIlIIIIlII// Patented Dec. 1, 1936 UNITED STATES PATENT OFFICE WORM GEAR BURNISHING MACHINE AND METHOD John M. Ghristman, Detroit; Mich., assignor to Packard Motor Car Company, Detroit, Mich., a corporation of Michigan This invention relates to a method of and appartus for forming and finishing worms, for instance worms such as are commonly employed in the steering gear of motor vehicles. In its preferred form the invention is applicable to the hour glass type of worm and is more particularly intended for the burnishing of hour glass worms which are designed for use with worm followers of the circumferentially ribbed roller type, this type of follower being frequently employed in lieu of the more conventional toothed worm gear or sector for the purpose of reducing friction in the steering gear.

It is of course essential that proper meshing of 5 the roller with the worm be ensured, and it is desirable for this purpose that the engaging surfaces of the worm and roller be of reasonably large area, and accurate and smooth finishing of both roller and worm is of the greatest importance. By practicing the present invention it is possible to produce reasonably true and sufiiciently smooth working surfaces on the worm and at the same time to harden and toughen these surfaces, the result of the burnishing operation being much more satisfactory in these respects than if grinding of the worm were resorted to. The invention contemplates the employment of one or more worm followers, preferably in the form of master rollers, each provided with one or more suitably hardened ribs adapted to mesh with the worm to be burnished, the worm and roller executing during the burnishing operation movements corresponding generally to those executed in actual use, pressure being simultaneously applied between the contacting surfaces.

In the formation of worms of the hour glass type a cutter is commonly employed, this cutter being rotated about an axis transverse to the axis of the worm, rotation being simultaneously im- 40 parted to the worm on its own axis. This is a conventional method of generating a worm of the hour glass type; Whether this method is employed or not, a worm generated by any of the more conventional methods is characterized by a thread of 45 varying helix angle. Thus adjacent the central portions of the worm where the diameter is least, the helix angle of the thread will be greatest, whereas towards either end of the worm, as measured axially, the helix angle will be less. Obvious- 50 1y this varying helix angle offers certain difiiculties when an effort is made to mesh with the worm a follower which is so constructed as to conform to any given helix angle, in other words, a follower which at the point of contact with the Worm 55 thread extends generally in the qii i tion of the thread and which is not so supported as to be capable of accommodating itself to some other thread direction.

Such is the case with a follower of the roller type and if it is preferred, as is'customarily the case, that accurate meshing of the roller shall be effected adjacent the central portion of the worm, the roller will not properly mesh at the end portions of the worm owing to the lesser helix angle of the worm thread. This difiiculty has been overcome to some extent in practice by'crowning the Workingsurfaces of the worm, but this is by no means a satisfactory solution of the problem. Thus while the crowning of the worm thread will afford improved contact between the worm and follower adjacent the ends of the worm, the crowned working surfaces do not afford adjacent the central portion of the worm a contact over a substantial area such as may be obtained with a flatworking surface. Itis'proposed as part of the present invention to burnish a crowned worm thread adjacent the central portion of the worm to flatten the surface to some extent to improve the nature of the contact and at the same time to: deform that portion of the thread adjacent the endsof the worm in a manner hereinafter more particularly pointed out.

The present invention incorporates certain improvements in the constructionshown and described in my prior application for'Letters Patent, Serial No. 9,093, filed March 2, 1935, of which this application is a continuation in part. In this prior application it is proposed to support the worm'follower for rotation on an axis which is spaced to a less extent from the worm'axis than is the axis of the cutting tool which generates the thread. It is also proposed to deform the thread, particularly adjacent either end of the Worm, to enable it to more readily cooperate with a follower designed more particularly to mesh properly with the thread at the central portion of the worm. 1

More specifically it is proposed to burnish a. worm of the character described by means of a hardened roller which is supported for traversing movement across the worm and in engagement with the worm thread about an axis which corresponds generally with the axisof the follower in the finished and assembled article and which describes an arc of less radius than that which is described by the generating tool. It is found that by this burnishing operation certain of the side faces of the thread will bealtered in shape and in general those portions'of the side faces of the thread on either side of the central portion of the worm which cooperate with the roller in the assembled gearing will be deformed and flattened so as to provide a broader area of contact between the roller and worm adjacent the ends of the latter.

If, for instance, a burnishing roller having one or more circumferential ribs is supported for rotation about an axis transverse to the worm axis and disposed on a line substantially perpendicular to the worm axis adjacent the central portion of the worm, but closer to the worm than the generating axis thereof, the central portion of the worm thread will be burnished with relatively small change in the contour thereof. For instance, if the working surfaces of the worm thread be slightly crowned, as is frequently done, a slight flattening of these crowned surfaces will be effected by the burnishing operation adjacent the central portion of the worm to improve the contact with the follower as hereinbefore suggested. Those portions of the thread toward the end of the worm, however, will be altered in contour in greater degree; in general the width of the groove between adjacent threads will be wider, and the worm will mesh more properly and over a greater contact area with a follower of the type which is particularly designed to accommodate that portion of the worm thread adjacent the center 0 the worm.

My prior application also discloses in the preferred embodiment of the invention a device for yieldingly urging the burnishing roller toward the Worm to provide the pressure necessary for the burnishing operation, the degree of pressure thus applied being variable whereby the nature and extent of contact between the worm and roller in the finished gearing may be controlled, and suggests the employment of positively acting means as a substitute for the yielding means shown and specifically described therein. It is this feature of the invention which is developed more particularly in the present application.

Thus it is proposed to provide means for con-v trolling the pressure between the burnishing roller and the worm which can be readily controlled so as to accurately predetermine and regulate the working pressure applied at different points in the length of the worm as the burnishing roller is traversed thereon. In this manner the extent to which the burnishing operation alters different portions of the worm thread may vary, and the clearance in the finished gearing between the worm and follower may vary at different points of traversal to suit the particular vehicle and steering mechanism in which the gearing is to be used.

The desirability of providing greater clearance between the follower and the worm toward either end of the latter is pointed out in my prior application. Thus as the roller passes toward either end of the warm in traversing the latter, the angular disposition of the circumferential ribs of the roller no longer conforms to the helix angle of the thread and it is necessary to provide greater clearance between the roller and the thread to prevent binding. It is furthermore desirable to provide this clearance toward either end of the worm so that when adjustments are effected to compensate for wear of the central portion of the worm which is subjected to the most use, there will be no binding of the roller with the worm near the ends of the latter, at which point less wear occurs. It is suggested that this clearance may be secured by the employment of a radius of swinging movement for the follower WP-P1! l than the radius of generation of the worm employed during the initial cutting operation. However, the degree of clearance necessary for proper steering will obviously vary in different vehicles. For instance, some vehicles exhibit much greater steering recovery, or tendency to return to the central steering position, than do other vehicles, and where excessive steering recovery is inherent it is necessary to check this tendency over a wider angle of steering movement by providing a closer fit between the cooperating elements of the gearing and less clearance between the follower and worm adjacent the ends of the latter than in vehicles exhibiting less steering recovery.

It is therefore an object of the present invention to provide a method and means for burnishing worms in which the pressure between the burnishing tool and the worm varies at different points of traversal. In the preferred form of the apparatus the desired control is achieved by the employment of a cam and the invention is illustrated as applied to a machine of the character described in my prior application in which the burnishing tool is supported for rotation on an axis spaced from the axis of the worm generating tool, whereby the side faces of the worm thread will be deformed by the burnishing operation in the manner hereinbefore briefly described. It will nevertheless be appreciated that in its broader aspects the present invention contemplates the application of means for effecting control of the burnishing pressure as herein described in connection with other burnishing processes and apparatus.

Further objects and features of the invention will be apparent from the following description taken in connection with the accompanying drawings, in which Figure 1 is a plan view of apparatus for carrying out the method of forming worm gears described in this application;

Figure 2 is an end elevation of the arrangement shown in Figure 1;

Figure 3 is a partial sectional view taken substantially on the line 33 of Figure 1;

Figure 4 is an illustration of a worm and follower in meshing engagement;

Figure 5 is a sectional View through a steering gear housing illustrating the worm and follower in the finished and assembled condition;

Figures 6, 7, and 8 are fragmentary sectional views taken respectively on the lines 6-6, 1-1, and 8-8 of Figure 4 and illustrating the thread contour;

Figure 9 is a section on the line 99 of Figure 3; and

Figure 10 is an enlarged diagrammatic representation of the surface of the cam shown in Figure 9.

For convenience in describing the invention reference is made to the preferred embodiment thereof and specific language is employed. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended but that various changes are contemplated such as fall within the spirit and scope of the appended claims.

Referring now to Figures 1 to 3 inclusive which illustrate one form of apparatus for carrying out the method described herein, it will be observed that a base In is provided, this base affording a support for a conventional head stock l3 and tail stock [2 in which a worm I5 is mounted for rotation on its own axis in the usual manner. As hereinbefore explained, this worm is of the type so generated, preferably by a single cutting operation only, that the helix angle of the thread is greatest at the central portion of the worm and decreases toward either end of the worm. Means may be provided for rotating the worm on its axis, a handle it associated with the head stock I3 being illustrated for that purpose.

Immediately above the worm and in engagement with the threads thereof is supported a follower l8 which in the preferred embodiment of the invention assumes the form of a circumferentially ribbed roller. While it is preferred to employ a roller having two circumferential ribs, either a less or greater number of ribs may be provided. The roller I8 is supported for rotation on its own axis in the head portion l9 of a carrier, the latter having a generally cylindrical shaft portion 24 which is supported for pivotal movement about an axis transverse to the worm axis. The worm axis and carrier axis are related as in the finished and assembled gearing, the carrier axis being located generally on a perpendicular erected on the worm axis.

adjacent the central portion of the worm and. being spaced to a less extent from the worm axis than the original axis of generation of the worm.

Referring more particularly to Figure 3, it will be observed that the shaft portion 24 of the carrier is journalled in a mounting member 2t! by means of one or more bushings 25, being retained in position by nuts 26 threaded on the remote end of the shaft portion 24 of the carrier. The mounting member 20 is in turn supported on the base In for rotation about an axis generally parallel with the Worm axis by means of a pin 22 carried in ears 2| which are preferably formed integrally with and are directed upwardly from the base.

Rotation of the mounting member 2|] in a clockwise direction about the pin 22 is limited by engagement of a radial cam 28 with a fixed abutment, for example the roller 29 which is shown as supported for rotation about an axis generally parallel to the axis of the carrier for the follower, a supporting bracket 36 secured to the base Ill being provided for this purpose. The cam 28 may be secured to the shaft portion 24 of the carrier by the nuts 26 and may be keyed against rotation thereon as indicated at 34. It will be appreciated that as the roller l8 traverses the worm l and the shaft portion 24 of the carrier is accordingly rotated about its axis, the cam 28 will react against the roller 29 to rock the mounting member 20 to a greater or less extent in a counterclockwise direction depending upon the configuration of the cam, the pressure with which the roller [8 is forced against the worm l5 being correspondingly varied.

It will be understood that during the operation of deforming or burnishing'the worm, the latter is in the relatively soft or unhardened condition, whereas the roller l8 has been hardened by any suitable treatment. Thus if the handle 16 be rotated, the follower I8 will traverse the worm from one end to the other and will readily burnish and flatten such portions as are directly engaged thereby, the necessary pressure being provided by the action of the cam 28. As hereinbefore explained, the roller I8, while permitted rotation on its own axis, is so mounted in the carrier I9 as to be incapable of rotation therein about any other axis, for instance about an axis intersecting the worm axis and the roller axis.

In other words, the roller l8 as shown morepar ticularly in Figure 4 is supported on an axis which is inclined with respect to any plane containing the worm axis, the ribs of the roller extending generally in the direction of the Worm thread adjacent the central portion of the worm.

It is thus not difiicult to ensure partial meshing of the follower and worm adjacent the central portion of the latter, but as hereinbefore explained, as the follower moves toward either end of the worm, the angular relationof the circumferential ribs of the follower fails toconform to the helix angle of the worm thread. This results in substantial reduction of the area of surface contact between the worm and follower, and toward the ends of the worm a spot contact rather than a surface contact is established. Under these circumstances both the end threads of the worm and the contacting portions of the roller tend to wear during use with the result that the roller eventually ceases to mesh accurately with that portion of the worm thread which is adjacent the center of the worm, at which point accurate meshing is most necessary.

However, when the hardened roller is supported as indicated in the accompanying drawings and is traversed across the relatively soft Worm, pressure being applied to maintain the contacting surfaces in intimate engagement, it is found that flattening of certain portions of the side walls of the worm thread follows, and this flattening of the thread walls is sufficient to ensure in the finished article a contact over a considerable area between the roller and the thread walls over substantially the entire length of the thread with consequent avoidance of rapid wear.

The manner in which the contour of the worm thread is altered may be understood from an inspection of Figures 6 to 8 inclusive of the drawings. It will be observed from Figure 6 that the portion of the worm thread adjacent the center of the worm, as measured axially, is burnished in such a way as to effect relatively small change in the shape of the worm thread as viewed in section, the operation involving principally a slight flattening of the working surface which as initially formed is preferably convexed or crowned to a small extent. Figures '7 and 8, on the contrary, indicate clearly that that face of the worm thread remote from the center is burnished to a greater extent adjacent the tip of the thread whereas that face of the worm thread directed toward the central portion of the worm is burnished to the greatest extent adjacent the base. In effect the width of the groove in the worm is increased so that the follower may be more readily accommodated to the worm and binding action between the follower and worm.

the radius of generation of the Worm employed during the initial cutting operation, the relationship between these rad ii being indicated generally in Figure 4 of the drawings. The point indicated therein as the center from which the radius of the cutter is measured indicates the position occupied by the axis of swinging movement of the cutter during generation of. the worm, and will be sometimes referredto hereinafter as the. generating axis. The natureof the burnishing action may be readily understood-if it is appreciated that by reason of the reduced radius of swinging movement of the roller, the latter is in effect rocked in a plane containing the axis of the worm and normal to. the axis of the roller carrier, increased pressure being thereby applied by the roller to the portions of the worm thread represented in dotted lines in Figure '7.

This flattening of the working surfaces of the worm thread toward either end of the worm is produced whether the pressure exerted by the burnishing roller is substantially constant as in the preferred form of the invention illustrated in my prior application, or whether the pressure is varied by the use of a cam device such as illustrated herein. Thus while the depth of the burnishing action may be increased by increasing the pressure, the effect on the shape of the working surfaces of the worm thread is essentially the same. It is, however, obvious that increase in the pressure applied by the burnishing roller at any given point along the worm will result in increased deformation of the worm thread at that point and will accordingly increase the clearance afforded at that point in the completed and assembled gearing. It has already been pointed out that vehicles having different steering action may require differences in the clearance between the cooperating elements of the gearing for most efiicient operation, and with the present construction these varying clearances may be readily formed by the burnishing operation, the increased expense of separate cutting or grinding operations and the loss of time required to perform the same being thereby avoided. For example, Figure 9 of the drawings illustrates one form of cam which has proven satisfactory in establishing the necessary pressure between the burnishing roller and the worm at different points along the length of the latter to produce a worm affording the varying clearance required for best results in a particular vehicle. It will be observed that this cam 28 is so formed that the least pressure is applied by the burnishing roller to the midpoint of the worm, the cam being shown in the position which it ot'cupies when the burnishing roller is in engagement with the central portion of the worm which receives the most wear in operation and which should mesh with the follower with the greatest accuracy. The portion of the cam periphery which is in contact with the abutment roller 29 is substantially flattened as indicated at 3| and the carrier for the follower may therefore swing away from the worm to the maximum extent permitted. As the worm is rotated by the handle N5, the follower moves toward either extremity of the worm, and one of the portions 32 of the cam 28 is moved into engagement with the abutment roller. Each of these cam portions 32 forms a gradual cam rise, the cam radius in-- creasing toward the points indicated at 33, the remainder of the cam being of constant radius. Thus as the follower moves away from the central portion of the worm, the applied pressure is increased and the deformation of the worm thread becomes greater. Consequently the clearance in the finished and assembled gearing will be greater toward the ends of the worm and yet the configuration of the cam may be such that at any given point in the length of the worm the gearing will mesh with sufficient accuracyto prevent improper or difiicult steering as the result of excessive steering recovery. Again, the clearance toward the ends of the worm may be made sufficient to permit such adjustment of the follower toward the worm as may be necessary on the occurrence of wear at the central portion of the worm without unduebinding at the less worn parts of the thread toward the worm ends.

As hereinbefore indicated, the working surfaces of the worm thread are often crowned as initially formedand it is preferred that the working surfaces of the burnishing roller |8 be substantially flat-sided so that adjacent the central portion of the worm, where accurate meshing is most important, the burnishing operation will result in slight flattening of thecrowned working surface of the worm thread to increase the area of contact between the worm andthe follower.

It will of course be appreciated that the form of cam shown in Figure 9 is merely illustrative and that the cam configuration will vary in accordance with the variation in clearness desired in the assembled gearing and will depend largely on the characteristics of the vehicle in which the gearing is to be used.

Figure 5 illustrates one mode of assembling the finished gearing. The worm l5 which has been formed, burnished, and hardened is keyed or otherwise secured to the lower end of the steering column 35 and is supported by means of bearings 36 in the conventional steering housing 38. A follower 40 in the form of a roller which is preferably similar to that used during the burnishing-operation is supported in a carrier 4| in meshing relation with the worm I5, the carrier 4| being in turn supported for rotation on an axis transverse to the worm axis and spaced to a less extent from the worm axis than the original generating axis of the worm. The manner in which the carrier 4| .is connected with the vehicle road wheels for steering purposes is of course conventional and forms no part of the present invention.

In the form of the invention shown in the drawings the worm thread is provided with a slightly crowned face, as indicated more particularly in Figures 6 to 8, while the burnishing roller and the roller in the assembled gearing are provided with substantially fiat side faces. will nevertheless be understood that the shape as viewed in section of the worm thread and the ribs on the roller may differ from that illustrated.

It has been found that practice of the present invention results not only in smoother operation of the gearing and in much less wear over an extended period of use, but actually simplifies to a considerable extent the forming and finishing of the worm and materially reduces the expense of production. Thus it is found that only one rough cut need be taken in generating the worm, the burnishing operation affording the smoothed polished surface which is desirable in gearing of this character while deforming the worm thread to improve the shape. Frequently only one traversing movement of the burnishing roller across the worm is required in order to attain the desired result.

While I have shown and described my invention as practiced with a master burnishing roller, it will be appreciated that the invention is in some aspects broader and may be carried out by the use of followers of other type or by the employment of any means which will effect the deformation of the worm thread in the manner herein described.

Having thus described the invention, what is claimed as new and desired to be secured by Let,- ters Patent is:

1. A method of making a worm of the hour glass type which comprises generating on a blank a thread having a helix angle varying axially of the worm, and thereafter burnishing the worm to deform the thread on either side of the central portion of the worm as measured axially to provide increased surface contact of the thread with a follower adapted for meshing with a thread of constant helix angle, while applying to the worm thread a burnishing pressure varying at points spaced axially of the worm. 2. A method of making a worm of the hour glass type which comprises generating on a blank a thread having a helix angle varying axially of the worm, meshing a circumferentially ribbed roller with the worm and displacing the roller about an axis transverse to the worm axis and lying in a plane substantially perpendicular to the worm axis at the center of the worm to burnish the worm, maintaining during the burnishing operation the initial inclination of the rib of the roller with respect to a plane containing the worm axis and intersecting the roller, and applying pressure of different degree between the roller and worm at different pointsalong the path of displacement of the roller.

3. A method of making a worm of the hour glass type which comprises generating on a blank a thread having a helix angle varying axially of the worm, meshing a circumferentially ribbed roller with the worm and displacing the roller about an axis transverse to the worm axis and lying in a plane substantially perpendicular to the worm axis at the center of the worm to burnish the worm, maintaining during the burnishing operation the initial inclination of the rib of the roller with respect to a plane containing the worm axis and intersecting the roller, and applying pressure of differentdegree between the roller and worm at different points along the path of displacement of the roller, the applied pressure being greater toward the ends of the worm than adjacent the center thereof.

4. A method of burnishing an hour glass worm generated by a cutting tool moved in an are about a predetermined center, which comprises meshing said worm with a circumferentially ribbed burnishing roller, and moving said roller about an axis lying in a plane substantially perpendicular to the worm axis at the center of the worm to cause the roller to traverse the worm, maintaining substantially an inclination of the rib of the roller corresponding to the helix angle of the worm thread adjacent the central portion thereof throughout the entire traversing movement of the roller, and applying pressure of different degree between the roller and worm atdifferent points along the path of displacement of the roller.

-5. A method of burnishing an hour glass worm generated by a cutting tool moved in an are.

about a predetermined center, which comprises meshing said worm with a circumferentially -ribbed burnishing roller, and moving said roller rib of the foll ercorrespoiiding to' the helix angle of the worm thread adjacent the central portion thereof throughout the entire traversing movement of the roller, and applying pressure of different degree between the roller and worm at different points along the path of displacement of the roller.

6. The method of increasing the area of contact between an hourglass worm and afollower which comprises supporting a hardened follower in meshing engagement with the unhardened worm for rotation about substantially the center of rotation of the follower in the finished assembled gearing, maintaining a fixed relationship between the follower and the worm con to cause the follower to traverse the worm thread, and applying greater pressure between the follower and the worm at points toward the ends of the worm than in the central portion thereof, whereby increased clearance may be provided in the finished assembled gearing as the follower moves away from the central portion of the worm. I

'7. The method of making worms of the hour glass type which comprises generating a worm thread on a blank corresponding to that which would be formed by rotating a worm and cutter at constant velocity about spaced, transversely disposed axes, the helix angle of said thread being less at points on either side of the central portion of the worm, meshing with the thread thus generated a follower having a portion for engagement with the thread. fashioned and maintained in substantial conformity with the helix angle of the worm at the central portion of the latter, traversing said follower across the worm, and applying greater pressure between the follower and the worm at points toward the ends of the worm than in the central portion thereof.

8. The method of making worms of the hour glass type which comprises generating a worm thread on a blank corresponding to that which would be formed by rotating a worm and cutter at constant velocity about spaced, transversely disposed axes, the helix angle of said thread-being less at points on either side of the central portion of the worm, meshing with the thread thus generated a follower having a portion for engagement with the thread fashioned and maintained in substantial conformity with the helixgreater surface contact with the follower, and

applying pressure between the worm and the follower which varies at points spaced in length of the Worm thread.

9. In apparatus-for burnishing worms of the hour glass type, the combination with a support for a worm, ofmeans for-rotating said Worm on the axis thereof, a, burnishing member sup- N ported on an axis transverse to the worm axis for traversing movement across said worm, and means for applying pressure between said burnishing member and worm of different degree at different points in the path of traversal of said member.

10. In apparatus for burnishing worms of the hour glasstype, the combination with a support for a" worm, of means for rotating said worm on the axis thereof, a'burnishing member supported on an axis transverse to the worm axis for traversing movement across said worm, and means for applying pressure between said burnishing member and worm of different degree at diiferent points in the path of traversalfof said member, said last named means comprising a cam device operable in timed relation with the traversing movement ofsaid member.

11. In apparatus for burnishing worms of the hour glass type, the combination with a support for a worm, of means for rotating said worm on the axis thereof, a burnishing member supported on an axis transverse to the worm axis for traversing movement across said worm, and means for applying pressure between said burnishing member and worm of greater degree as the member moves towa'rdthe ends of the worm than when said member is traversing the central portion of the worm.

12. In apparatus for burnishing worms of the hour glass type, the combination with a support fora worm, of means for rotating said worm on the axis thereof, a burnishing member supported on an axis transverse to theworm axis for traversing movement across said worm, and means for applying pressure between said burnishing member and worm of different degree at different points in the path of traversal of said member, the degree of pressure being increased as the member moves in one direction across the worm from the central portion of the latter.

13. In apparatus for burnishing worms of the hour glass type, the combination with a support for a worm, of means for rotating said Worm on the axis thereof, a burnishing member supported on an axis transverse to the worm axis for traversing movement across said worm, means for applying pressurebetween the worm and burnishing member, and means for automatically varying the pressure at points spaced in the path of traversing movement of said member.

14. In apparatus for burnishing Worms of the hour glass type, the 'combinationwith a support for a worm, of means for rotating said worm on the axis thereof, a burnishing member supported on anaxis transverse to the worm axis for traversing movement across said wormand at a less distancetherefrom than the generating axis of the worm, means for applying pressure between the worm and burnishing member, and means for automatically varying the pressure at points spaced in the path of traversing movement of said member.

15. In apparatus for burnishing worms of the hour glass type, the combination with a support for a worm, of means for rotating said worm on the axis thereof, a burnishing member supported on an axis transverse to the worm axis for traversing movement across said worm and at a less distance therefrom than the generating axis of the worm, means for applying pressure between the worm and burnishing member, and means for automatically varying the pressure at points spaced in the path of traversing movement of said member in such manner that the degree of applied pressure increases in either direction from the central portion of the worm.

- thread heli 16. In apparatus for burnishing worms ofthe hour glass type, the combination with a support for a worm, of means for rotating said worm on the axis thereof, a burnishing member supported on an axis transverse to the worm axis for traversing movement across said worm and at a less distance therefrom than the generating axis of the worm, means supporting said burnishing member in a position corresponding to the helical angle of the thread at the central portion of said worm during the entire traversing movement of said member, and means for applying pressure between said burnishing member and worm of different degree at different points in the path of traversal of said member. a

17. In apparatus for burnishing worms of the hour glass tpe formed by generation to afford a angle greater in the central portion than at the ends, the combination with means supporting said worm for rotation on the axis thereof, of a burnishing member having a circumferential rib, means supporting and maintainingsaid member in engagement with said thread in a position corresponding to the helix angle of the thread in the central portion of said worm, said last named means being supported for rotation about an axis located substantially in a perpendicular from the central portion of the worm axis, and means for applying pressure to said last named means to urge said burnishing member into engagement with said Worm, said last named means acting automatically to vary the applied pressure at different points spaced in the direction of the worm axis.

18. In apparatus for burnishing worms of the hour glass type, the combination with a circumferentially ribbed roller, of means for supporting the roller and a worm. to be burnished in meshing relation, means for rotating the worm on the axis thereof and for swinging the roller about an axis differing from the generating axis of the worm but disposed closer to the worm, and means for simultaneously applying pressure between the contacting surfaces of said worm and roller, said last named means acting automatically to increase the pressure as the roller swings away from the central portion of the worm in either direction.

19. The method of forming a worm of the hour glass type which comprises efiecting a single rough cutting' operation, thereafter burnishing said worm to substantially alter the thread shape adjaoent the ends of the worm without materially altering the thread shape adjacent the central portion of theworm, and applying to the worm in the burnishing operation a pressure varying at points spaced in the length of the worm thread.

20. The method of forming a worm of the hour glass type which comprises cutting a blank to provide a worm'thread having slightly crowned working surfaces, and thereafter burnishing the worm to substantially alter the thread shape adjacent the ends of the worm and to slightly flatten the working surface adjacent the central portion of the worm and applying to the worm in the burnishing operation a pressure varying at points spaced in the length of the worm thread. I

, JOHN M. CHRISTMAN. 

